Do You Feel Lucky?

Casinos and hotels must keep the power flowing

Any industry that depends on electricity to generate revenue knows that the quality of the electrical supply isn’t always what is needed to keep the revenue stream flowing. Depending on how large that revenue stream is determines how much effort is made in trying to keep it flowing, 24/7.

The hospitality/gaming/resorts industry is no exception. Walk into a casino at three in the morning and you would think that it is the afternoon. Having the stage lights flickering during a Broadway show would be distracting to both the performers and the audience (and could induce nausea or worse). Even to those potential winners in the long lines waiting for tickets to the Mega-Millions lottery draw would be quite upset if a power interruption resulted in the ticket machine going down, having them miss out on getting their lucky tickets. Here’s a take on which type of mitigation equipment is effective for which type of power-quality phenomena.

To determine what equipment to use is usually dictated by monitoring at the point of utilization to determine what is the quality of supply versus the susceptible load. The types of protection can be broken into two basic categories—those that suppress the power-quality (PQ) phenomena and those that correct the phenomena. Among the most common equipment used in the first category is TVSS or transient voltage surge suppressors. These are often found in outlet strips used to plug in PCs, monitors and printers and may be a mixture of MOV (metal oxide varistors), avalanche diodes, capacitors and other components that “snub” or “clamp” the over-voltage condition associated with positive impulsive transients.

Take some precautions when using TVSS devices. First, they should be UL-listed and have an integral fuse, since some of the components are designed to fail in a short-circuit state if the transients are either too large or too repetitive. Second, they can only work on the positive portion of the transient. They do little for negative transients, such as notches, and provide no help for sags (or interruptions), which are normally far more common occurrences than transients.

Another type of mitigation equipment in this category is a harmonic filter, which are tuned circuits for specific harmonics that are considered to be too large in magnitude and cause problems in the facility, such as overheating transformers or other electromagnetic equipment. If the loads in the facility change quantity or frequency content (such as adding adjustable speed drives), it may require a redesign of the filter.

The second category of mitigation equipment includes equipment that add energy back into the voltage waveform to make up for either a decrease from a sag, or a total lack of voltage, from an interruption—for example, an automatic tap-changing transformer that returns the voltage level to the nominal voltage. Such equipment is also useful for overvoltage conditions. However, the speed at which the control circuits react is often multiple cycles, which may be too slow to keep equipment operational without some additional short-duration ride-through equipment.

Perhaps the most commonly used equipment of this category is the UPS or uninterruptible power supply. UPS units are either of the continuous operation or bypass types. In the former, the AC supply is used to charge the batteries, which are used to continually convert the DC voltage back into AC voltage to power the loads. The latter uses a bypass switch to let the AC voltage pass straight through, except when the control circuits determine that the input AC voltage is too low. When running off the batteries, the overall efficiency is lower than in bypass; however, there is no transfer time, which may be a factor in the latter type. Batteries do have maintenance requirements, and there are other types of storage devices, such as flywheels or superconductive magnets, but battery-based UPS units are far more common.

UPS equipment can only supply as much energy during sags or interruptions as was stored while the AC voltage was “normal.” Since the majority of sags are less than 10 cycles (in most surveys), this isn’t normally a problem. For longer-duration interruption protection, a back-up generator can be used by itself or in conjunction with UPS equipment. The generator may take several seconds to get up to the rated speed that is required to provide adequate voltage magnitudes at the rated frequency. A local generator is usually not as “stiff” a source as the generators on the interconnected electric utility grid. This can result in deeper sags for the same loads starting up, as well as changes in frequency when large loads are energized. And these, too, require a storage of energy—often diesel fuel—that need some adequate refill planning for sustained interruptions like August 14, 2003.

Facilities that want to have “six 9s” of up-time (up 99.9999% of the time) often combine mitigation equipment. This doesn’t make them immune to PQ problems; proper integration is crucial. But with the right equipment, you can change the probability of a power-quality-related process interruption from luck, to an event as rare as winning the lottery.